Stents are used in conjunction with a medical procedure known as balloon angioplasty to restore blood flow through obstructed or partially obstructed arteries. In an angioplasty procedure, a balloon catheter is inserted into an artery through a small incision and is advanced to the site of an arterial lesion via a catheter. Subsequently, the balloon catheter is inflated to compress the accumulated atherosclerotic plaque against the artery wall, thereby restoring blood flow through the vessel. In some cases, the expanded artery will collapse after deflation of the balloon catheter or will slowly narrow over time, a process referred to in the art as restenosis.
Stents are small, tubular structures which are implanted in the vessel to provide mechanical support to the arterial wall. The use of stents has been shown to result in a lower incidence of restenosis which has consequently lead to a more widespread use of stents in the treatment of vascular disease.
Stents are fabricated from a variety of metals, among other types of materials, including, for example stainless steel, most commonly 316L stainless steel, and nickel-titanium alloys such as NITINOL. Stent materials are selected based on mechanical properties, corrosion resistance and vascular compatibility.
Typically, the stent manufacturing involves machining a specially designed stent pattern into thin walled tubes or flat sheets of the desired metal. This cutting is preformed utilizing laser beam machining (LBM), electrode discharge machining (EDM) or chemical dissolution (“chemical milling”). In the case of flat sheets, the machined part is then rolled and welded into a tubular shape.
The LBM technique cuts the material with a focused, high-energy beam of light. The EDM technique utilizes an electrical spark discharge to cut in a similar manner. The chemical milling technique consists of masking the metal with a chemically resistant material and then dissolving away the exposed metal with a chemical solution. The masking is then removed resulting in the machined stent.
In both the LBM and EDM techniques localized heating of the machined part occurs, resulting in melted and oxidized metal byproducts along the cut surfaces. Additionally, all three machining techniques result in a coarse, rough surface and sharp edges. Many processes have been developed to remove the byproducts created by the machining operations (“descaling” or “cleaning”). A separate process is used to polish the stent surface and round sharp edges (“electropolishing”).
Descaling methods are known in the art for many metals. One example is immersion of stainless steel or nitinol alloys in a heated mixture of hydrofluoric acid and nitric acid. Another example is the electrolytic treatment of stainless steel in a sulfuric acid solution. In these operations, the machining byproducts are removed leaving behind a clean part that is free of remnant debris from the cutting operation. Alternately mechanical grit blasting is sometimes used to remove oxides from titanium alloys. Other procedures for descaling are well known to those skilled in the art.
The principles of electropolishing, particularly with regard to stainless steel alloys, are also known in the art. Electropolishing typically involves dissolving and smoothing the metal surface by electrolysis. Using this method, an item to be electropolished is immersed in an electrolyte which typically comprises a conductive, acidic solution. A counter electrode is also immersed in the solution and is typically connected to the negative terminal of the power supply (creating a cathode). Typically, the positive terminal of the power supply is attached to the part, thereby completing the electric circuit. An appropriate electrical potential is applied between the workpiece and the cathode and current flows.
Upon the passage of electric current through the electrolyte, metal is dissolved from the anode surface creating a resistive film on the surface of the workpiece. Protrusions on the anode surface dissolved faster than depressions producing a smoother surface. Examples of electropolishing processes include: dissolution of stainless steel in phosphoric acid and sulfuric acid; dissolution of titanium (or alloys) in perchloric acid and methanol, and dissolution of stainless steel in glycerol and sulfuric acid. Additionally, gold can be electropolished through dissolution in a solution comprised of potassium cyanide, potassium tartrate, potassium ferrocyanide, disodium hydrogen phosphate, and ammonia hydroxide.
The now polished item is typically immersed in nitric acid. The nitric acid is effective in removing certain metallic oxides, salts or other impurities such as mixed iron or other reactive oxides or particulates. The goal is to ensure high Cr/Fe ratios and homogenous surface chemistry free of residual metal compounds formed during the electropolishing operation that may compromise the surface with respect to biocompatibility. The result is a clean, biocompatible, corrosion resistant surface.
Many metal medical implants contain active transition metals, which form passive oxide surfaces. The stable metal oxide is resistant to corrosion. Processes for descaling and electropolishing these alloys are known in the art.
More recently, however, other properties such as the radiopacity of the material, i.e. the x-ray absorption properties, has become a consideration in order to observe the stent during both the medical procedure and during follow up exams using fluoroscopy to visualize it. Consequently, new metal alloys have emerged in order to improve the radiopacity of the stent. Examples of such alloys include, for example, those of cobalt, chrome, and tungsten such as L-605, for example.
Yet another way in which the radiopacity may be improved is to add a noble metal to an existing alloy, such as the addition of platinum to 316L stainless steel such as that found in conunonly assigned copending U.S. patent application Ser. No. 10/112,391, the entire content of which is incorporated by reference herein.
This approach has resulted in a marked improvement in the fluoroscopic visualization of stents. However, the addition of noble metals to the stent material complicates the manufacturing process due to the chemical inertness of noble metals. Thus, currently employed descaling and polishing operations are neither efficacious nor effective for uniformly dissolving alloys having noble metals. Consequently, there remains a need in the art for an improved method for descaling and electropolishing alloys having noble metals as part of the alloy composition.